Responses to deviants are modulated by subthreshold variability of the standard.

Auditory mechanisms automatically detect both basic features of sounds and the rules governing their presentation. In the oddball paradigm, the auditory system detects the sameness (or no-variability) rule when the same reference tone is consistently repeated. We used two oddball protocols, the classical one with a fixed reference and a modified one with a jittered reference, to determine whether the auditory system can detect subthreshold violations of sameness. We found that the response to the repeated standard was not modified by the small jitter. However, the response to the frequency oddball was smaller under the jittered protocol, indicating hypersensitivity to sameness. The sensitivity to jitter was largest when the oddball deviated by 8%, was smaller for 40%, and disappeared at 100% deviation, indicating that sensitivity to sameness is context dependent; namely, it is scaled with respect to the overall range of stimuli.